I use the wedding one because most of my cakes are 2 layers and that is a nice size piece.

Both of these charts are based on a 2 layer cake. If you're only making a 2" tall (one layer) cake then divide the number in half. The difference between the charts is the serving size. The wedding chart is the industry standard for serving sizes of 8 cubic inches (1x2x4 or 2x2x2)